In my experience it takes less time to reheat a cooked item than it does to cook it. This is true for every single different "type" of cooked item I can think of. (Meat, soup, pasta, beans, etc etc).

It's quite common for me to use the microwave to reheat things, and that might lead me to be biased in thinking that it's faster because the microwave itself is often the fastest way top reheat something, but this observation isn't true just for microwaving. It doesn't even seem to matter on the method of reheating, as I can reheat something faster if I use the same method of as I did to cook it (e.g. by frying).

Note that I always check the temperature of something I've reheated via a food-probe, so I'm also not making a mistaking of cooking something to 70C and then reheating to 45C etc.

So:

Is it always faster to reheat something than it was to cook it, or are their exceptions?

Note: Googling this question just results in endless results of people asking how to reheat food X. I don't know if I'm a google bubble or if no-one out there has asked this question before? If I were to guess I'd say it's because you need more energy to do whatever it is that happens to proteins/starch when they cook, and one that's done you need less energy to simply heat it. Or something?
– PodApr 18 '19 at 13:26

5 Answers
5

"Cooking" is often a chemical process. Denaturing proteins, gelatinization, causing chemical reactions like browning, or even causing state changes like evaporation.

In many cases for these reactions to happen, we need to overheat the food. (Cook it and let it rest to cool off back down to undo some of the changes that were made and/or bring it back down to a reasonable temperature to eat). This is true when grilling meats, frying, baking bread, and lots of other types of cooking.

Other times, we need to bring something to temperature and hold it there for some period of time. This holds for extracting collagen, starch gelatinization (eg, cooking pasta, potatoes, etc.) but also just waiting for flavors to transfer in soups and similar dishes.

With warming, you're just adding enough heat to it to move it a few degrees, but you're not typically trying to change the state of the food, so less total energy is needed.

Now, it is always faster to reheat vs. cook things? For the most part it's true, but I suspect that there would be an edge case out there. Something that's cooked from room temperature, but then stored chilled and the chilling causes issues (like retrogradation in starches, maybe?) that make them more resist than reheating.

I think your fourth paragraph should be highlighted more. That's the real answer to the question. Some of the heat energy is going into state changes, so it's not all being used to actually change the temperature of the food.
– GentlePurpleRainApr 18 '19 at 14:48

10

To add to the last point, consider that reducing 100ml from a 1L pot of liquid requires evaporating that much water. Evaporating water requires an enormous amount of energy - for 100ml it works out to 226kJ of energy. If you were reheating the 900ml of liquid left, from 4C in the fridge to 70C (65C delta-T) for eating, you require 4.2J/gC, or about 250kJ. So reheating 900ml of cold soup takes the same amount of energy as reducing 100ml from 1L of soup which has already been heated to 100C. State changes consume large amounts of energy and cooking is all about state changes.
– J...Apr 18 '19 at 15:37

4

In addition to the fourth paragraph, (usually) when you cook some water will evaporate, meaning that when you reheat there's less water to warm up than when you cooked it in the first place, making it even faster to reach the temperature required.
– Alexandre AubreyApr 18 '19 at 16:13

3

@J...: are non-water state changes in cooking usually also endothermic? Maillard reaction? Breaking down collagen? Reactions can be exothermic but still not occur at room temperature (e.g. oxidation/combustion of wood), so requiring holding at higher temp doesn't prove that heat energy is going anywhere except being lost to the surrounding air / room, and carried off by evaporation except in a covered pot. Your example of reducing a liquid is a great example of a clearly endothermic process that's common in cooking, though.
– Peter CordesApr 19 '19 at 15:59

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@PeterCordes Agreed. Putting a lid on the pot goes a long way, for example. Just containing the heat lost due to evaporation (and some negligible radiant cooling) you can leave a stew simmer on very low heat where it would have needed medium heat without a lid - clearly it's not the cooking reactions that are consuming the energy, as you say. I think it's probably safe to say that most energy lost in cooking is due to water evaporation, simply because water is ubiquitous in food and its enthalpy of vapourization is so ridiculously high.
– J...Apr 19 '19 at 17:53

This is because when you're cooking some foods you're not just heating it up. A lot of foods are boiled, not because they need to be heated up, but because they need to absorb water. We just boil the water because that makes the hydration go a lot faster (the high temperature is also needed to break down some of the starches, for more info, see here).

With soup it should take about the same time, if you don't care about dissolving/softening the vegetables into the soup. That also takes time, with vegetables the chemical reaction involved is mainly breaking down the pectin that holds the cells of the vegetable together.

With meat, dissolving/denaturing the collagen (stuff that holds everything together) into gelatin also takes time. Also you want a different temperature for reheating than frying because with meat you want a nice crispy brown outside (Maillard reactions), and for that you need far higher temperatures than the inside of your meat.

It doesn't even seem to matter on the method of reheating, as I can reheat something faster if I use the same method of as I did to cook it (e.g. by frying).

Note that I always check the temperature of something I've reheated via a food-probe, so I'm also not making a mistaking of cooking something to 70C and then reheating to 45C etc.

That is not strictly possible. If you are imparting the same amount of heat energy to the same thing at the same rate in the same controlled environment, then the resulting temperature must necessarily be identical.

If you are ensuring a consistent overall temperature resulting from the same source, then the time difference arises because you are heating different things.

One likely culprit would be water that escaped as steam during during cooking or evaporated during/after, which reduces the mass you are heating the second time around. Water is also one of the slowest things to heat, because it has one of the highest specific heat capacities amongst common substances. This alone would result in a very noticeable difference in many types of food.

Because heating up is merely rising the temperature of a body and how much its temperature change depends on its specific heat. The sane is for the complex mix of the various items in the pot as we are speaking about kitchen.

Cooking involves a number of physical and chemica processes, each of which takes time. Is this taking time the major difference, that is why I've decided to add this answer alongside the others. They aren't wrong at all, just in a way incomplete. Cooking must be accomplished, and that will be the case anyway, see just here below.

Most of these process require heat as well, that is energy must be given to the system. So the pot must stay on stove (or the meat on the grill, etc.) longer.

Independent of this energy requirement, which for some chemical transformations can be even positive (ie the process releases energy and not vise versa), chemical reactions go faster higher the temperature is.
For instance, pasta could be cooked at lower than boiling point, just it will take longer. This is why pressure cooking is somehow faster as well less energy consuming.

edited.

Specifically to question number 1, yes is at least in principle possible that a cooked item takes longer to be heat as compared to heat the original item. If cooking involved water intake, the specific heat of the cooked item might be bigger, for instance. An example is likely pasta. I would expect that it takes longer to bring a cooked spaghetto to 100 °C than doing it with a raw one. But this analysis is certainly out of the kitchen (fine measuring, ad hoc experiments, way of heating....), as probably we never put raw vs cooked spaghetti on a hot plate and measure how long it takes for them to reach the wanted T.

[the other answers] aren't wrong at all, just in a way incomplete I agree, but I also think your answer is incomplete! :) Is it possible to give some examples of a physical of chemical process that happens when cooking, but not when heating? e.g. converting raw chicken protein into cooked chicken protein?
– PodApr 24 '19 at 11:20

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Pod. Happy that you get my point. The only thing that I don't understand of your comment is that I do not see how to get into heating without starting cooking as well. I could well denature egg proteins, or even an egg, but this happens from a certain T up. As far eadible items are taken as a whole, cooking requires heating, which can be semantic. Cheese affinage it is something else. I am satisfied that you understood the nuance, but @Echox say the same although in a more spartan way. I go up voting his/her A too. Tell me if I misunderstood your comment.
– AlchimistaApr 25 '19 at 6:40

To cook, you heat something and let it stay hot until it get cooked.
To heat, you just heat it a bit until you can eat it.

So even it you want to eat it as hot as its cooking temperature (which you won't in most cases, with a good 100°C margin), you just ignore all the "cooking time" after you reached the right temperature.

When you mention a 100C margin, I think you're talking about oven air temperature, not food temperature. If you stick a probe thermometer in your food and heat it to ~140 Celsius, you'll drive out all the water by boiling it off on the way to that temp, and proteins will break down leaving any meat basically inedible way beyond the point of overcooking to a crumbly dry disaster.
– Peter CordesApr 19 '19 at 15:52

Does this question really call for a scientific explaination ?. Yes, as that's what I'm interested in. Otherwise I would have asked "How do I heat up food?????".
– PodApr 24 '19 at 11:17

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What I mean is that your original question can be translated as "Why is something that I heat for a long time takes longer than something I heat for a short time" and doesn't need any kind of physical or chemical explaination. But maybe what you really wanted to know is what happens when you cook something and why does it needs to stay at a certain temperature for a long time but then your questions would need editing.
– EchoxApr 24 '19 at 12:27